1. Field of the Invention
The present invention relates to a rare earth magnet. More specifically, the present invention relates to a technology to improve magnetic properties, in particular, thermal stability of a rare earth magnet.
2. Description of the Related Art
Bonded magnet, which is prepared by compounding magnet powder in a resin or a rubber, is widely used in various applications such as electric appliances and car parts due to freely controllable form and easiness in improving dimensional accuracy.
In recent years, miniaturization and weight reduction of electric appliances and car parts have been demanded, and for bonded magnet that is used for these applications, it has been strongly demanded to realize miniaturization without lowering in their magnetic properties. To respond to this demand, improvement of magnet performance is required. More specifically, it is required to improve residual magnetic flux density and coercive force as well as heighten maximum energy product.
As a measure to enhance magnetic properties, improvements in magnet composition of magnet powder and magnet structure have been proposed. With regard to magnet composition, a ferrite type bonded magnet using a magneto-plumbite type ferrite had been widely used. However, the ferrite type bonded magnet has such magnetic properties that residual magnetic flux density Br, coercive force iHc and maximum energy product (BH)max are comparatively lower. Due to the reason, an Nd2Fe14B type bonded magnet has been spread. The Nd2Fe14B type bonded magnet is produced using a quenched thin ribbon that is obtainable by feeding a molten raw alloy on a rotating roll.
With regard to an improvement of magnet structure, an exchange spring magnet, in which a permanent magnet phase (a hard phase) and a soft magnet phase (a soft phase) are coexisting in nano-size, attracts attention as a novel magnet material. The exchange spring magnet can improve magnetic properties due to high magnetic flux density as a whole magnet because it contains a soft phase with a high magnetic flux density.
Enhancements of magnetic properties are progressing by the above-described improvements, whereas a problem has been pointed out that rare earth bonded magnet is inferior in thermal stability. A magnet applied in a site, where the magnet is exposed to such a high temperature environment as for automobile driving system, is particularly required to have superior thermal stability and a small irreversible demagnetizing factor.
As a measure to improve thermal stability of the rare earth bonded magnet, a technology to improve thermal stability by forming a plurality of convex streaks or grooves on the surface of magnet powder has been disclosed (JP-3277932). However, in consideration of use in such a high temperature environment as for automobile driving system, a further improvement in thermal stability is required.
As other measures to improve thermal stability of the rare earth bonded magnet, a method for improving thermal stability by controlling magnet composition, production conditions of a quenched thin ribbon and heat treatment conditions of a quenched thin ribbon has been developed by one of the present inventors [Hiroshi Yamamoto and Kazuma Takahashi, Journal of The Magnetics Society of Japan, Vol. 27, No. 5, pp. 698-703 (2003)]. In the above-mentioned Reference, for a Pr—Fe—Co—Ti—Si—B type of exchange spring magnet, influences of each of magnet composition, roll circumferential speed in producing a quenched thin ribbon, heat treatment conditions of a quenched thin ribbon and heat treatment time of a quenched thin ribbon on magnetic properties have been studied by varying these factors.
However, in consideration of use in applications where the magnet is exposed to a high temperature environment, a further improvement of thermal stability is preferable.